In typical radio data communication systems, each radio transceiver is configured for use with a single antenna. However, single antenna configurations often prove inadequate for maintaining wireless communication because of inherent antenna orientation and performance limitations. To solve such inadequacies, some radio transceivers have been configured with a second antenna having alternate orientation and/or performance characteristics. With two antennas, the transceiver can select one of the two antennas based on the receipt of incoming communications.
More specifically, in current dual antenna designs, the selection process (referred to hereinafter as an "antenna diversity protocol") involves the selection of either a best antenna or a satisfactory antenna. To select the best antenna, the transceiver listens to transmissions using the first antenna and then the second antenna. Thereafter, for further communication, the transceiver selects the antenna that yielded the best reception performance. In a satisfactory antenna diversity protocol, the transceiver first listens with the first antenna. If reception proves satisfactory, the transceiver selects the first antenna for further communication. Otherwise, the transceiver listens using the second antenna, and, with satisfactory reception, the second antenna is selected.
With single antenna configurations, a transceiver adds a specific preamble bit sequence to each packet of data to be transmitted. Knowing the specified preamble in advance, another transceiver that successfully receives such a transmission can easily detect and lock on to the preamble portion of the transmission. Having identified the preamble portion, a receiving transceiver can be fairly confident that it can successfully receive the subsequent information portion of the transmission. If instead the preamble cannot be discerned, a receiving trsceiver concludes that it cannot reliably receive the subsequent data portion of the transmission.
To accommodate preamble identification, the preamble must be of such content and duration as to permit reliable identification by a receiving transceiver. For example, some current preambles consist of a "101010 . . . " bit pattern sequence, because such a sequence can be rapidly detected. Depending the specific sequence (the content), a transceiver will require a certain duration of time (i.e., a certain number of received preamble bits) to identify a preamble. Thus, if the preamble portion of the packet is made too short in duration, transceivers would not be able to make an accurate identification before the data portion of the transmission begins. If the preamble portion is too long, the overhead associated with sending the preamble becomes highly undesirable due to the inherent decrease in data transmission throughput. Additionally, longer overall packet size (caused by longer preamble lengths) leads to a higher likelihood of reception failure. In many system protocols a longer preamble increases the chances for collisions between two or more units competing with the channel. Thus, the preamble portion of the packet must be of only such duration as to permit reliable preamble identification by the transceivers.
To accommodate best or satisfactory diversity protocols, transceivers have been configured to perform their antenna selection during the preamble portion of a transmitted packet. Unlike single antenna configurations, however, transceivers having multiple antennas must be capable of identifying the preamble of a transmission a plurality of times.
For example, using the best antenna diversity protocol, a transceiver having access to two antennas must first attempt to identify the preamble with the currently selected antenna. After either identifying or failing to identify the preamble, the transceiver switches to the other antenna to also attempt to identify the preamble. If the transceiver fails to identify the preamble with either antenna, the transceiver does not attempt to receive the data portion of the transmission because of reliability concerns.
If only one of the two antennas yielded a satisfactory identification of the preamble, the transceiver utilizes that antenna (the "successful antenna") to attempt to receive the subsequent data portion of the transmission. If the successful antenna happened to be the one currently selected, the transceiver merely listens for the end of the preamble and beginning of data. However, if the successful antenna happened to be the previously selected antenna, the transceiver must switch antennas and re-identify the preamble before the data begins. Otherwise, the transceiver cannot count on reliable receipt of the data portion of the transmission.
If both of the antennas yield a satisfactory identification of the preamble, the transceiver selects that antenna which exhibits the best signal quality (the "best antenna"). This may also require that the transceiver switch antennas and re-identify the preamble if the best antenna is not currently selected.
Thus, using the best antenna diversity protocol with two antennas, to accommodate worst case scenarios, the preamble must be about three times longer in duration than that necessary for a single antenna, i.e., three time periods for preamble identification attempts (hereinafter "observation windows") plus two antenna switching time periods.
Similarly, to support the worst case scenario, the satisfactory antenna diversity protocol using two antennas requires a preamble of about two times the duration of that needed for single antenna configuration. Specifically, where the first antenna cannot satisfactorily identify the preamble, the preamble length must include: 1) a first time period for attempting to identify the preamble with the first antenna (a "first observation window"); 2) a second time period during which the transceiver switches to the second antenna; and 3) a third time period for attempting to identify the preamble with the second antenna (a "second observation window").
Causing further problems for the diversity protocols, transceivers occasionally miss the beginning of a preamble period, and, therefore, unexpectedly encounter the end of the preamble and beginning of the data. Often this unexpected encounter results in the diversity protocol's failure to complete the antenna selection process within the remaining preamble time period, resulting in a lost transmission. Factors causing a transceiver to miss the beginning portion of a preamble include: 1) collisions with another transmission; 2) other types of interference or noise; or 3) an inopportune beginning of reception after the start of the preamble transmission.
Because transceivers do not detect that they have missed part of the preamble, they may invoke a diversity protocol that is destined to fail. Best antenna diversity protocols are extremely vulnerable in such situations, frequently missing transmissions that a single antenna systems would receive.
Although the satisfactory antenna diversity protocol provides the most immunity to an unexpected preamble end, the satisfactory protocol does not always use the "best" antenna available for a given communication. As a result, transmission failures after successful antenna selection are more likely to occur with the satisfactory protocol than with a best antenna diversity protocol.
In addition, many current transceivers implement various forms of multipath compensation (also referred to herein as "adaptive equalization") using the preamble. As with diversity protocols, each multipath compensation technique requires a certain duration of preamble for successful application, with some (typically better) techniques taking much longer than others. If the beginning of a preamble is missed, such techniques fail upon encountering an unexpected end of the preamble.
Factors for selecting a preamble bit sequence to provide optimal adaptive equalization often conflict with factors for providing rapid preamble identification. Thus, in current preamble bit sequences, compromises are made. As a result, either the ease of preamble identification or adaptive equalization or both suffer.
More specifically, IEEE 802.11 compatible communication networks provide a preamble of a duration proposed to be sufficient to support antenna diversity and multipath compensation. The specified preamble bit sequence (or preamble content) consists of a known uniform sequence. For example, a "dotting pattern" of 1-0-0 . . . is specified for Frequency Hopping (FH) communication, and a "marking pattern" of 1-1-1-1 . . . is specified for Direct Sequence (DS) communication. Both of these simple bit sequence patterns have been selected because they are easily recognized by transceivers as the preamble. More complex sequences having longer-repeating or non-repeatable bit patterns are much harder to recognize. However, neither of these bit sequence patterns provide an indication of the beginning of a preamble or warn of a preamble end.
With the currently specified uniform preamble sequence, a receiver, attempting to identify the preamble for the first time, has no knowledge of whether it has synchronized to the first bits of the preamble, the center, or the end. Therefore, the worst possible timing relationship between start of transmission and antenna sampling must be assumed in designing a diversity protocol. For example, to support two antennas, the best antenna protocol requires only two or three observation periods to make a decision. Even so, to accommodate at least a percentage of worst case scenarios (wherein the beginning of the preamble has been missed), five of such observation periods might be included in the preamble length. Yet where antenna scanning and start of transmission happen to coincide, the last two or three periods would constitute unnecessary overhead. The timely receiver is forced to wait through such periods not knowing when to expect the preamble end.
Moreover, the subject of antenna selection has been discussed in detail in a recent paper entitled "Carrier Sense with Diversity Modifier FH PHY" by Dean Kawaguchi, IEEE P802.11-94/70, March 1994. Kawaguchi discloses methods for performing two-antenna selection diversity, but does not consider alternatives such as n antenna selection diversity, adaptive equalization, maximum ratio combining, or RAKE approaches for direct sequence. All of these techniques generally benefit from training during the preamble period, and all are subject to the aforementioned limitations faced by antenna diversity and multipath compensation protocols.
Therefore, an object of the present invention is to provide transceivers in a wireless communication network with the capability of optimally using a preamble of minimal duration for all preamble purposes.
It is a further object of the present invention to provide an optimal "n" antenna diversity protocol which does not suffer from unnecessary preamble overhead, and which implements the advantages of both the satisfactory and best antenna diversity protocols.
It is another object of the present invention to prevent a transceiver from encountering an unexpected end of a preamble, regardless of whether the beginning of a preamble transmission has been missed.
Another object of the present invention is to inform transceivers of their location in the preamble so that they can most appropriately select from their available antennas without unexpectedly encountering a preamble end.
Still other objects of the present invention will become apparent with further reference to the remaining specification, claims and related figures.